Allow infallible boolean extractors to return true type

[LPTK]

The following code (here in Scastie) does not currently work:

sealed abstract class Maybe[A]
final case class Just[A](a: A) extends Maybe[A]
class Empty[A] extends Maybe[A]
object Empty {
  def apply[A](): Maybe[A] = new Empty[A]
  // def unapply[A](e: Empty[A]): Boolean = true  // ok
  def unapply[A](e: Empty[A]): true = true
}

object Test {
  val a: Maybe[Int] = Just(2)
  def main(args: Array[String]): Unit = a match {
    case Just(2) => true
    case Empty() =>
    // ^ Boolean(true) is not a valid result type of an unapply method of an extractor()
  }
}

Though it should arguably compile and raise an exhaustivity warning (because it does not match Just(n) for n != 2).